The present invention relates to vertical-cavity surface-emitting lasers (VCSELs), and more particularly to a method and apparatus for controlling and stabilization of polarization in such devices.
A typical structure of a conventional vertical-cavity surface-emitting laser (VCSEL) is illustrated in FIG. 1. Specifically illustrated is a simplified cross-sectional view of a portion of a VCSEL 10. VCSEL 10 is fabricated on a semiconductor substrate 12, more particularly e.g. a gallium arsenide substrate. A first mirror region, typically a stack of distributed Bragg reflectors 14, comprised of a plurality of alternating layers 16 is positioned on a surface 15 of semiconductor substrate 12. The plurality of alternating layers 16 of first stack of distributed Bragg reflectors may be formed of n-doped aluminum arsenide material and a n-doped gallium aluminum arsenide material. There is next fabricated a cladding region 26 on a surface of the first stack of distributed Bragg reflectors 14, an active region 20 disposed on cladding region 26, and a cladding region 27 disposed on a surface of active region 20. A second mirror region, typically a stack of distributed Bragg reflectors 22 is positioned on a surface of cladding region 27. The second stack of distributed Bragg reflectors 22 is formed of a plurality of alternating layers 23, more specifically, for example, alternating layers 24 and 25 of a p-doped aluminum arsenide and a p-doped gallium aluminum arsenide. The second stack of distributed Bragg reflectors 22 is followed by a one-half wavelength aluminum gallium arsenide contact layer 28. Contact layer 28 is p-doped to 1e19 cmxe2x88x923 or higher. Finally, a very thin gallium arsenide cap layer 30 is positioned on a surface of contact layer 28. Cap layer 30 is very thin, more specifically on the order of 100 xc3x85 thick. Cap layer 30 is p-doped to 1e19 cmxe2x88x923 or higher.
The active region 20 is typically constructed from one or more quantum wells of InGaAs, GaAs, AlGaAs, (Al)GaInP, GalnAsP or InAlGaAs, or is a bulk material active region.
It should be noted that VCSEL 10 is not shown to scale in FIG. 1. In particular the mirror regions and active regions have been expanded to provide clarity in the drawing. In practice, the thickness of the substrate 12 is typically 150 xcexcm compared to about 10 xcexcm for the mirror and active regions.
After processing of the VCSEL 10, it may be cleaved from the original substrate 12, and mechanically coupled to another substrate, called a mounting substrate. It may, however, also be kept on the original substrate 12, which is from then on also called (with regard to the present invention) a mounting substrate.
Because VCSELs are highly symmetric semiconductor lasers that emit light in a direction perpendicular to the plane of the quantum well(s), neither waveguiding nor gain anisotropy inherently exist. As a result VCSELs suffer from poor polarization stabilization. Switching between two modes of linear and orthogonal oriented polarization (PSxe2x80x94polarization switching) and mode hopping between these two states is often observed and leads to mode partition noise and to degradation of the optical systems based on VCSELs when polarization is important.
Different methods to force a preference to one of the two orthogonal linearly polarized states have been suggested. Most of them require a modification of the VCSEL structure by introduction of strains during the layer deposition/growth process or a complex additional processing of the VCSELs. The VCSEL performance could be degraded as a result.
EP-0924820 describes a VCSEL assembly in which the polarization is locked to a specified direction by externally introducing stress with a predetermined direction and magnitude. The VCSEL assembly includes a mounting substrate having top and bottom surfaces, the VCSEL being mechanically coupled to the top surface of the mounting substrate. This mounting substrate is selectively etched at the bottom side, whereby a groove or trench is formed which weakens the mounting substrate along a specific direction. A bonding agent such as solder with a different thermal expansion from that of the mounting substrate is used to bond the mounting substrate to another surface. When the solder joint is applied, the solder is molten at a temperature of typically 275xc2x0 C. As the solder cools and solidifies, the solder joint shrinks thus applying force on the VCSEL assembly and forcing it to bend in a direction determined by the slot axis.
It is a disadvantage of the method device described in EP-0924820 that the polarization locking is based on different thermal coefficients of the bonding agent and the VCSEL assembly. This means that the locked device is inherently temperature dependent, and that the danger exists that the polarization of the emitted light changes with changing temperature. Furthermore, an additional etching step in the mounting substrate is needed. Additionally, the thermal treatment required to attach the solder may cause damage to the VCSEL assembly. Finally, it is difficult to form the slot or trench in third party devices so that chip packagers are dependent upon the original chip manufacturer to form the groove or trench.
It is an object of the present invention to provide an efficient and economic method for control and stabilization of the polarization state of the light emitted by any type of VCSEL as well as a VCSEL device which has a stabilized polarization state.
The present invention may provide a method for stabilizing the polarization of light generated by a VCSEL assembly comprising a light generation region of a VCSEL, the VCSEL being mechanically coupled to a mounting substrate. The method comprises the steps of: mechanically coupling the mounting substrate to a second substrate which is coextensive or larger in area than the mounting substrate, and applying uniaxial strain to the light generation region of the VCSEL by means of external strain applied to the mounting substrate by the second substrate.
The present invention may also provide a VCSEL device having a stabilized polarization of light generated by a VCSEL assembly comprising: a light generation region of a VCSEL, the VCSEL being mechanically coupled to a mounting substrate, a second substrate being coextensive or larger in area than the mounting substrate in at least one direction, the mounting substrate being mechanically coupled to the second substrate, and means for applying strain to the light generation region of the VCSEL by means of external strain applied to the mounting substrate by the second substrate.
The invention provides an economical way to apply uniaxial strain to a VCSEL which is stable over time and can be made independent of temperature changes, e.g. as caused by loading.